feature: splitlen - count HEX as its underlying length

[roycewilliams]

When using splitlen, it would be useful to have an option for the 'true' length (in bytes) to be used, taking HEX encoding into account.

A crude approach, rather than explicitly converting from HEX, would be to detect HEX and calculating the 'true' length as (rawlength-6)/2 (assuming no malformed HEX, depending on how strict the HEX detection/validation is.